The invention relates to a process for forming thin film cermet resistance patterns, and more particularly to such a process wherein the cermet is uniformly etched with good undercut control and minimal attack on the underlying substrate and the masking photoresist layer.
Cermet is a metal and ceramic material having a high resistance and low capacitance and which is easily trimmed by laser annealing. These characteristics make cermet an excellent thin film resistor material for GaAs integrated circuits. However, cermet is a difficult material to pattern in the extremely small geometries needed for large-scale integration. One reason for this is that chemicals which are natural etchants of cermet tend to also attack the masking photoresist layer and the underlying substrate at high rates.
U.S. Pat. No. 4,081,315 describes a particular cermet etching technique wherein a layer of molybdenum is used on the cermet layer to provide a good adherent for the photoresist layer which is deposited on the molybdenum layer. The exposed areas of the cermet are then etched with hot phosphoric acid at a temperature of around 180.degree. C.
Although this technique produces excellent resolution without damaging the other parts of the circuit, further improvements have been made by way of this invention. This invention provides a process for etching cermet which provides these characteristics by using an aluminum layer deposited on the photoresist layer and the exposed areas of the cermet and etching the cermet with a mild solution of hydrofluoric acid. Moreover, the photoresist layer is deposited directly on the cermet eliminating the molybdenum layer.